In a typical video display system, images can be created by either emitting or modulating light. The light forms picture elements (pixels), which, when viewed together with other pixels, form an image. The pixels in an image will typically have a variety of colors and/or intensities, with image quality being dependent upon a number of different intensity levels the pixels are capable of displaying. A binary spatial light modulator (binary SLM), such as a binary micromirror device (DMD), is digital in nature and is not capable of emitting light with different intensity levels. However, there may be other SLMs that can be digital in nature but are not binary. Instead, the binary SLMs will typically rely on a pulse width modulation (PWM) scheme to create light at various intensity levels by rapidly turning a light modulator on and off. The integration of the rapidly switching light by the eye provides an illusion of multiple intensity levels.
Being mechanical devices, there is a limit to how rapidly the light modulator can be turned on and off. For example, in a DMD, the time that is required to turn the light modulator (a mirror in the DMD) on and off corresponds to moving a mirror from the off state to an on state and then back to the off state. This time can be dominated by a time that the mirror (micromirror) takes to settle to a stable state after moving. This translates to a minimum amount of light that can be emitted within a single frame time. The minimum amount of light corresponds to a lowest intensity level that can be produced by the binary SLM and can be referred to as a bit-depth of the video display system. In general, the smaller the minimum amount of light, the higher the bit-depth and the finer the image quality produced by the video display system.
One technique that can be used to reduce the minimum amount of light produced by the binary SLM is to make use of a neutral density filter (NDF) to modulate the light for the short duration light pulses. The NDF can have different densities and therefore can attenuate the light to different levels.
Another technique that can be used to reduce the minimum amount of light produced by the binary SLM is to use dynamic aperture technology. Dynamic aperture technology makes use of adjustable apertures to reduce the intensity of the light.
One disadvantage of the prior art is that the use of the NDF causes loss of light during the entire time of reduced illumination. This time is far greater than the switching on/off time of the mirror. This loss of light results in a reduction of overall system brightness.
A second disadvantage of the prior art is that the use of the NDF or the dynamic aperture technology to modulate light amplitude can require modifications to existing binary SLM products and technologies, which can require significant redesign and redevelopment. This can lead to the expenditure of a large amount of time and money.
A third disadvantage of the prior art is that both the NDF and the dynamic aperture technology techniques are mechanical techniques, which also have physical limits on a minimum amount of light that can be emitted. Therefore, it may not be possible to reduce the minimum light intensity to a desired level if the physical limits are too high. Additionally, mechanical techniques may not be able to provide a desired level of flexibility when it comes to exactly producing a needed level of light.